A known walking operator type lawn mower has a belt transmission structure and a tension clutch structure as shown in FIG. 5. A transmission belt 12 is wound around a drive pulley 10 rotatable in opposite directions by an engine (not shown) and a driven pulley 11 connected to a rear wheel 2. A first tension pulley 13a and a second tension pulley 13b are arranged outside passages of right and left belt portions of the transmission belt 12. The first and second tension pulleys 13a and 13b are attached to a first tension arm 14a and a second tension arm 14b which are independently oscillatable on a transverse axis P1. A tension spring 15 extends between the first and second tension pulleys 13a and 13b.
The drive pulley 10 as shown in solid lines in FIG. 5 is standing still. When the drive pulley 10 is rotated counterclockwise as shown in FIG. 6, the righthand belt portion 12a is stretched under tension. Consequently, the first tension pulley 13a is pushed rightward, whereby the second tension pulley 13b is also drawn rightward. This results in the lefthand belt portion 12b being drawn inward. In this way, the entire transmission belt 12 is maintained in tension to be free from slips, and power is transmitted to the driven pulley 11.
A tension clutch structure for breaking the power transmission through this belt transmission structure will be described next. As shown in FIG. 6, a first auxiliary arm and a second auxiliary arm are attached to distal ends of the first and second tension arms 14a and 14b to act as oscillatable actuating members 16a and 16b. The first and second auxiliary arms 16a and 16b are interconnected at distal ends thereof by a pin. A rod 23 is connected to the pin also.
With this construction, when the rod 23 is pulled downward, the first and second tension pulleys 13a and 13b are spread sideways apart through the first and second auxiliary arms 16a and 16b. As a result, the transmission belt 12 is relaxed to produce a declutched state (as shown in two-dot-and-dash lines in FIG. 5).
According to the known construction described above, the first and second tension arms 14a and 14b and the first and second auxiliary arms 16a and 16b are freely oscillatable by the respective pin connections. Therefore, even when the first and second tension pulleys 13a and 13b are sideways spread apart to produce the declutched state, shakes or vibrations of the vehicle tend to cause sideways swings together of the first and second tension pulleys 13a and 13b as spread apart.
Such sideways swings would reinstate the tension pulleys 13a and 13b in tension, whereby the power is inadvertently transmitted from the drive pulley 10 to the driven pulley 11.